This invention relates to a method for making an improved phosphor. This invention also relates to the use of the improved phosphor in an information display device having photochromic glass.
Optical information storage and display devices of the type including a cathode ray display tube have recently come into accepted commercial use. In one such device, a cathode ray beam is addressed across a phosphor screen which in turn excites a plate of photochromic glass. Ultraviolet emission from the phosphor screen darkens the photochromic glass. The image created in the photochromic glass is read out with a suitable optical system. Systems of this type are described in Italian Pat. No. 845,551 -- Megla and Ludovici (corresponding to U.S. application Ser. No. 677,185 filed Oct. 23, 1967).
The phosphors used in these systems have, in general, been developed for other purposes, such as television and fluorescent lamps. In general the efficiency of emission in the visible spectrum is important. On the other hand, in information display systems using photochromic glass the ultraviolet emission is particularly important because light of this wavelength darkens the photochromic glass.
Existing commercial processes for making phosphors emphasize the production of phosphors in large batches. While such techniques are suitable for the production of phosphors for use in particular applications such as television tubes, they are not suitable for the production of phosphors for photochromic glass display systems. For example, present commercial processes for producing BaSi.sub.2 O.sub.5 :Pb result in a product which does not have the desired high efficiency for use in these systems. The BaSi.sub.2 O.sub.5 :Pb produced commercially is therefore best used in other applications, for example, as a black light phosphor in fluorescent lamps.